As per the wiki, "Due to being inherently fireproof, magma cubes can spawn on magma blocks."
But they don't.
When I noticed the problem, I created a new world and went into Creative mode on Simulation Distance of 4.
I built a 9x9 platform out of magma blocks, up in the air of a basalt delta, with more than 4 blocks of air above the platform.
I moved 44 blocks horizontally away from the platform farthest edge of the platform. No other spawnable areas were within 44 blocks of me.
I waited for 10 minutes. Magma cubes never spawned on the platform.
To be sure mobs were spawning, I then built another 9x9 platform out of Netherrack, next to the magma block platform with a one-block gap of air between the platforms.
Within a couple of minutes, Magma Cubes were spawning on the Netherrack platform, but still not the Magma Block platform.
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Can confirm, affects 1.20.60.26 and 1.20.51.
From my observations, this probably also applies to ghasts.
How to reproduce:
Import
After joining the world, just stand still and wait for the mobs to spawn
Observed results:
Ghasts and magma cubes appear to only spawn on the netherrack platform.
Expected results:
They should also be able to spawn on the magma block platform.
Affects 1.20.41 playstation and realms